Continuous method for purifying carbon nanotube

ABSTRACT

Provided is a continuous method and apparatus for purifying carbon nanotubes. Carbon nanotube is fed together with solvent into a preheater via a heat exchanger to produce a carbon nanotube mixture. The carbon nanotube mixture is preheated at 100 to 370° C. Then, the carbon nanotube mixture is purified in a purifying reactor under a subcritical water condition of 50 to 400 atm. The resulting purified product is cooled down to 0 to 100° C. and depressurized into 1 to 10 atm by feeding the purified product into a cooling down and depressurizing part via the heat exchanger. Finally, the cooled and depressurized product is recovered.

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

The present invention claims priority of Korean Patent Application No.10-2008-0095856, filed on Sep. 30, 2008, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a continuous method of purifying carbonnanotubes, and more particularly, to a continuous method of purifyingcarbon nanotubes under a sub-critical water or supercritical watercondition.

2. Description of Related Art

A structure of the carbon nanotube (hereinafter, referred to CNT) wasdiscovered firstly in 1991; and manufactures, physical properties andapplications thereof have been accomplished actively. Further, it isconfirmed that the CNT is prepared if transition metals such as Fe, Ni,and Co are added upon electric-discharging. The full-scale research isnot initiated until considerable samples are produced by means of alaser vaporization method in 1996. Such CNT is in a shape of hollow tubehaving a graphite surface rolled up in a nano-sized diameter. At thistime, the electric property of the CNT is classified into conductor orsemi-conductor in accordance with degree and structure with which thegraphite surface is rolled up. Further, the CNT can be classified into asingle-walled carbon nanotube SWCNT, a double-walled carbon nanotubeDWCNT, thin multi-walled carbon nanotube, a multi-walled carbon nanotubeMWCNT, and roped carbon nanotube in accordance with the number of thegraphite walls.

In particular, the CNT is superior in mechanical intensity andelasticity, and has chemical stability, environmental friendliness, aswell as electrical conductivity and semi-conductivity. Further, the CNThas a diameter of 1 nm to several tens nm and a length of several μm toseveral tens μm so that it is greater than any exiting materials ofwhich aspect ratio amounts to about 1,000. Further, since itsspecific-surface area is very large, it is under the spotlight invarious fields such as future-generation information electronicmaterial, high-efficiency energy material, high-capacity complexmaterial, and environmental friendliness material.

However, it is difficult to utilize electric, mechanical and physicalproperties of the CNT due to impurities such as carbon substance,amorphous graphite, and alpha carbon besides the CNT which are preparedduring the manufacturing process of the CNT. Therefore, it is necessaryfor a continuous method of purifying a large amount of CNT in order toenlarge the practical range in various uses while supporting theproperties of the CNT. The technique of purifying the CNT is exemplifiedas a method of purifying the CNT using an acidic gas pyrolysis in Koreanpatent No. 2001-0049298, a method of purifying through the pyrolysisusing an oxidizer in U.S. Pat. No. 5,641,466 and a method of purifyingat a higher temperature using the oxidizer in Japanese Patent No.1996-12310.

However, in cases of the above techniques, the time needed for heattreatment is long and steps up to an acid-treatment process arecomplicated, which results in consuming too much time.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to providing acontinuous method of purifying carbon nanotubes under a sub-criticalwater or supercritical water condition using a continuous apparatuswithout separate purifying processes.

To achieve the object of the present invention, the present inventionprovides a continuous method of purifying carbon nanotubes comprising afirst purifying step for injecting a carbon nanotube liquid mixturecontaining an oxidizer into a purifying reactor under a sub-criticalwater or supercritical water condition at a pressure of 50 to 400 atmand a temperature of 100 to 600° C. to obtain a purified product.

Further, the present invention provides a continuous apparatus ofpurifying carbon nanotubes comprising a mixing unit for forming a carbonnanotube solution by allowing the carbon nanotubes to be mixed with asolvent containing water using a circulation pump; a pre-heater forheating a carbon nanotube liquid mixture formed by allowing thepre-heated carbon nanotube solution to be in contact with an oxidizerand mixed with the oxidizer while the carbon nanotube solution isinjected at a pressure of 50to 400 atm; a first purifying-reactor inwhich the liquid mixture is injected under a sub-critical water orsupercritical water condition at a pressure of 50 to 400 atm and atemperature of 100 to 600° C.; a de-pressurizing unit forde-pressurizing the purified product up to 1 to 10 atm via a coolingapparatus that cools the purified product to a temperature of 0 to 100°C.; and a product storage vessel for recovering the productde-pressurized by the de-pressurizing unit.

Further, the present invention provides the carbon nanotubes purifiedaccording to the continuous method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of continuous method of purifying carbonnanotubes according to one preferable embodiment of the presentinvention.

FIG. 2 is a flow diagram of continuous, apparatus of purifying carbonnanotubes including a filtering apparatus according to one preferableembodiment of the present invention.

FIG. 3 is a flow diagram of continuous apparatus of purifying carbonnanotubes including a filtering apparatus according to one preferableembodiment of the present invention.

FIGS. 4 a and 4 b are Scanning Electron Microscope SEM pictures of thecarbon nanotubes which are not purified for each magnification accordingto a comparative example 1.

FIGS. 5 a and 5 b are Scanning Electron Microscope SEM pictures of thecarbon nanotubes which are purified for each magnification according toEmbodiment 2.

FIGS. 6 a and 6 b are Scanning Electron Microscope SEM pictures of thecarbon nanotubes which are purified for each magnification according toEmbodiment 1.

FIGS. 7 a and 7 b are transmission Electron Microscope TEM pictures ofthe carbon nanotubes which are not purified for each magnificationaccording to a comparative example 1.

FIGS. 8 a and 8 b are transmission Electron Microscope TEM pictures ofthe carbon nanotubes which are purified for each magnification accordingto Embodiment 2.

FIGS. 9 a and 9 b are transmission Electron Microscope TEM pictures ofthe carbon nanotubes which are purified for each magnification accordingto Embodiment 1.

DETAILED DESCRIPTION OF MAIN ELEMENTS

10: circulation pump

20: CT solution high-pressure infusion pump

30: oxidizer high-pressure infusion pump

40: heat exchanger

50: acid solution high-pressure infusion pump

60: cooling apparatus

70: filtering pressure control apparatus

80: pressure control apparatus

100: mixing unit 200: pre-heater

310: first purifying-reactor

330: second purifying-reactor

410: first filtering unit

430: second filtering unit

500: filtrate storage vessel

600: de-pressurizing unit

700: product storage vessel

DESCRIPTION OF SPECIFIC EMBODIMENTS

The advantages, features and aspects of the invention will becomeapparent from the following description of the embodiments withreference to the accompanying drawings, which is set forth hereinafter.In the drawings, it is noted that like components or parts representpossibly like reference numerals. Upon explaining the present invention,the specific explanations on the related known function or structure areomitted in order to make the subject matter of the present inventionobvious.

The terms of “about”, “substantially” and so on used in thespecification are used as a numerical value or a meaning closed to thenumerical value when a tolerance of production and material inherent inthe meaning mentioned is presented, and used to prevent consciencelessinvaders from using the teachings in which correct or absolute numeralvalues are disclosed to help the understanding of the present invention

FIG. 1 is a flow diagram of continuous purification method of carbonnanotubes according to one preferable embodiment of the presentinvention. Referring to FIG. 1, the purification of carbon nanotubesaccording to the present invention may be processed in a carbon nanotubesolution production step S100, a carbon nanotube solution injecting stepS200, an oxidizer injecting step S300, a pre-heating step S400, a firstpurification step S500, and a cooling step S800; and further an acidsolution injecting step S600 and a second purification step S700 may beprocessed after the first purification step S500, and a filtering stepS911, a product recovering step S913, and de-pressurizing step S915 orthe de-pressurizing step S931 and a product recovering step S933 areprocessed after the cooling step S800.

FIG. 2 is a process view showing a continuous apparatus of purifyingcarbon nanotubes in which a filtering apparatus is included according toone preferable embodiment of the present invention. Referring to FIG. 2,the continuous apparatus of purifying carbon nanotubes according to thepresent invention includes a mixing unit 100, pre-heater 200, a firstpurifying-reactor 310, a first filtering unit 410, a second filteringunit 430 and a filtrate storage vessel 500, and a secondpurifying-reactor 330 may be further included next to the firstpurifying-reactor 310.

FIG. 3 is a process view showing a continuous apparatus of purifyingcarbon nanotubes according to one preferable embodiment of the presentinvention. Referring to FIG. 3, the continuous apparatus of purifyingcarbon nanotubes in which a de-pressurization tub is included accordingto other example of the present invention may further include a mixingunit 100, a pre-heater 200, a first purifying-reactor 310, ade-pressurization tub 600 and a product storage vessel 700, and a secondpurifying-reactor 330 may further be included next to the firstpurifying-reactor 310.

The present invention may include a first purification step in which thepurified product is formed in the purifying-reactor by injecting thecarbon nanotube liquid mixture including the oxidizer under asub-critical water or supercritical water condition at a pressure of 50to 400 atm and a temperature of 100 to 600° C. and a second purificationstep in which inorganic matters is to be removed by causing the firstpurified product to be reacted with the acid solution.

The carbon nanotube liquid mixture containing the oxidizer is formed bywhich the oxidizer is injected to make it contact with the carbonnanotube solution while the carbon nanotube solution is injected intothe pre-heater 200 located at a front end of the first purifying-reactor310. At this time, the carbon nanotube liquid mixture is injected intothe pre-heater 200 and then is undergone through the pre-heating stepS400 at a temperature of 200 to 370° C.

Specifically, the continuous purification method of the carbon nanotubeis characterized in that the carbon nanotube and a solvent is circulatedby a circulation pump 10 in the mixing unit 100 to produce the carbonnanotube solution in a carbon nanotube solution production step S100,the carbon nanotube solution is injected into the pre-heater 200 locatedat a front end of the first purifying-reactor at a pressure of 50 to 400atm by the CNT solution high-pressure infusion pump 20 in a carbonnanotube solution injection step S200 while the oxidizer is injected bythe oxidizer high-pressure infusion pump 30 at a pressure of 50 to 400atm in an oxidizer injecting step S300, and the liquid mixture of thecarbon nanotube solution and the oxidizer formed by contacting theoxidizer to the carbon nanotube solution is injected into the pre-heater200 and undergone through the preheating step S400 at a temperature of200 to 370° C.

In the carbon nanotube-solution production step S100, the carbonnanotube solution containing the carbon nanotube and the solvent isinjected into the mixing unit 100 and produced by the circulation pump10 before the liquid mixture of the oxidizer and the carbon nanotube isproduced. The solvent contained in the CNT solution necessarily containswater and can be selected from a group consisted of aliphatic alcohol ofC1-C20, carbon dioxide and a combination of them.

The CNT is preferably selected from a group consisted of asingle-walled, a double walled, a thin multi-walled, a multi-walled,roped and a combination of them.

Further, the CNT is contained with at least 0.0001 parts by weight ofthe solvent and preferably 0.001 to 19 parts by weight. If the CNT isless than 0.0001 parts by weight, the recovery amount of the CNT is toosmall.

As the CNT solution is injected into the pre-heater 200 at a pressure of50 to 400 atm through the CNT solution high-pressure infusion pump 20,the CNT solution is contacted to the oxidizer injected using theoxidizer high-pressure infusion pump 30 at a pressure of 50 to 400 atmso that the CNT solution is mixed with the oxidizer at a front end ofthe heat exchanger 40, and then the liquid mixture of them is injectedinto the pre-heater 200 and pre-heated at a temperature of 100 to 370°C.

If the pressure is less than 50 atm when the CNT solution and theoxidizer are injected through the high-pressure infusion pump, it isdifficult to allow the CNT solution and the oxidizer to be injected intothe pre-heater 200 and the first purifying-reactor. On the other hand,if the pressure is more than 400 atm, the energy loss is caused due tohigher pressure.

The carbon nanotube solution further contains a nitro compound of achemical equation 1.R—(NO_(x))_(y)  [Chemical equation 1]Where, R is alkyl group of C₁-C₇ or an aryl group of C₆-C₂₀, x and y areintegers of 1 to 3 independently. The nitro compound is preferably nitromethane, nitro ethane or nitro propane.

The nitro compound is contained at 0.00001 to 3 mol/L, in which it isimpossible to purify the carbon nanotube efficiently if the nitrocompound is less than 0.00001 mol/L; and it is impossible to make suredesirable yields since the carbon nanotube can be damaged while theinorganic matters and amorphous carbon are removed if it is more than 3mol/L.

The preheating tub 200 is to pre-heat the liquid mixture in order tomake uniform a temperature of the first or the second purifying-reactors310 and 330 before treating the liquid mixture under the sub-criticalwater or supercritical water condition that is mentioned later.

Therefore, the pre-heater 200 is provided with the heat exchanger at afront end thereof and is responsible for pre-heating the liquid mixtureof the CNT and the oxidizer. The heat exchanger 40 is responsible forprimarily lowering the temperature before ultimately cooling thepurified product under the sub-critical water or supercritical watercondition to prevent the energy loss from being consumed upon subsequentcooling. If the temperature is less than 100° C., the temperature isfurther raised at the threshold condition so that the energy loss cannot be prevented, and if it is more than 370° C., the energy needed toincrease the temperature above preheat efficiency is even increased sothat there is no need to provide the heat exchanger.

Meanwhile, the oxidizer can be selected from a group consisted ofoxygen, air, ozone, hydrogen peroxide, nitro compound, nitric acidoxidizer, and a combination of them. The oxidizer can be contained at a0.001 equivalent to 10 equivalent in proportion to carbon equivalent ofthe CNT within the carbon nanotube liquid mixture.

The impurities such as nanocarbon, amorphous carbon and alphacarbonwhich were contained in the CNT solution before the CNT is oxidized dueto the oxidizer can be oxidized and eliminated. This is because the nanocarbon, amorphous carbon, alpha carbon has a high reactivity with theoxidizer in comparison to the CNT so that the reaction rate with theoxidizer is very quick. That is, the impurities are eliminated due to adifference in the reaction rate of particle sizes.

Therefore, the CNT is not uniformly oxidized with the oxidizer to causeimpurities not to be eliminated and the purification rate to be low ifthe oxidizer is injected at less than 0.001 equivalent based on the CNTcarbon, while the purification efficiency is not improved as much if itis injected at more than 10 equivalent.

The carbon nanotube liquid mixture preheated via the pre-heating stepS400 is carried to the first continuous purifying-reactor 310, where thefirst purification step S500 is processed for the carbon nanotube underthe sub-critical water or supercritical water condition of 50 to 400atm. At this time, the temperature under the sub-critical water orsupercritical water condition is preferably 100 to 600° C.

The pressure of the sub-critical water condition is preferably 50 to 260atm and more preferably 60 to 260 atm. Further, the temperature ispreferably 100 to 380° C. and more preferably 200 to 350° C. At thistime, the treatment time is preferably processed for 1 to 30 minutes,and more preferably for 5 to 15 minutes.

Meanwhile, the pressure in the supercritical water condition ispreferably 150 to 400 atm, and more preferably 210 to 300 atm. Further,the temperature is preferably 350 to 600° C., and more preferably 370 to500° C. At this time, the treatment time is preferably processed for 1to 30 minutes, and more preferably for 5 to 15 minutes.

Due to such sub-critical water or supercritical water condition, sincethe oxidizer is quickly reacted with the impurities such as nanocarbon,amorphous carbon and alpha carbon, it is possible to remove theimpurities via oxidation in a short time. Therefore, the purificationefficiency can be differentiated due to a difference in reactivity ofthe oxidizer and the impurities

The selection under the sub-critical water or supercritical watercondition is to control the purification rate, which means a temperatureor a pressure condition represented above.

The continuous method of purifying carbon nanotubes according to thepresent invention can further include the second purification step S700in which the first purified product is reacted with the acid solution toremove the inorganic matters. Since the second purification step canfurther purify the inorganic matters as well as impurities within thecarbon nanotube to improve the purification effect, it is possible toobtain high quality samples applied to FED, LCD backlight,high-integrated memory device, fuel cell and the like which require theCNT of high purity.

After the first purified product via the first purification step S500 isinjected into the second purifying-reactor 330, the acid solution isinjected into the second purifying-reactor 330 using the acid solutionhigh-pressure injection pump 50 at a pressure of 50 to 400 atm and atemperature of 100 to 600° C. to remove the inorganic matters such asmetal or catalyst in the acid solution injecting step S600. If thepressure at which the acid solution is injected is less than 50 atm, itis difficult for the acid solution to be injected into the secondpurifying-reactor 330, and if it is greater than 400 atm, the energyloss is caused due to a higher pressure and the purification rate is nolonger improved.

Herein, the continuous method of purifying carbon nanotubes according tothe present invention allows the acid solution to be injected into anentrance portion of the second purifying-reactor under the sub-criticalwater or supercritical water condition.

The acid solution is reacted with the metal inorganic matters containedin the catalyst used at the time of the carbon nanotube manufacture toform salt which is then melted into the solution, so that the inorganicmatters are removed.

Therefore, the acid solution can contain any one acid selected from agroup consisted of nitric acid, hydrochloric acid, phosphoric acid,sulfuric acid and a combination of them, and nitric acid or phosphoricacid is preferable.

The continuous method of purifying carbon nanotubes is characterized inthat the acid solution is injected at 0.00001 to 3.0M, and preferably0.005 to 1.0M. If the acid solution is injected at less than 0.00001M,the impurities such as inorganic matters are not sufficiently removed,and if it is injected at greater than 3.0M, the removal efficiency arenot improved as much, which results in waste materials.

The present invention provides a continuous method of purifying carbonnanotubes which includes a cooling step S800 cooling the first or thesecond purified product at 0 to 100° C.; a filtering step S911 after thecooling step; a recovery step S913 recovering the filtered product; ade-pressurizing step S915 de-pressurizing the product at 1 to 10 atmafter recovering it.

In the acid solution injecting step S600, the acid solution is injectedinto the second purifying-reactor 330 located in a rear portion of thefirst purifying-reactor 310 by the acid solution high-pressure infusionpump 50 to purify the metal inorganic matters of the first purifiedproduct at a temperature of 200 to 300° C. The heat exchanger 40 whichis provided at a front end of the pre-heater 200 and used to pre-heatthe CT solution is reused for firstly cooling the first or secondpurified product discharged from the second purifying-reactor 330 to bea temperature of 100 to 200° C., thereby preventing the energy loss.

The first or second purified product is firstly cooled by the heatexchanger 40 and then cooled up to a temperature of 0 to 100° C. via thecooling apparatus 60. The cooling temperature is preferably controlledat a temperature of 20 to 50° C.

The filtering step S911 can be processed via the filtering units 410,430 which have high-pressure filters with a void of 0.001 to 10 μmconnected in parallel to be operated in a switching manner, in order tofilter the first or the second purified and cooled product. It ispossible to recover the purified CNT of solid state via the filteringstep S911.

The filtering units 410, 430 cause the product to be divided intofiltrates 411, 431 and purified CNT filtered product 413, 433, and thefiltrates 411, 431 are de-pressurized at a normal pressure state via afiltering pressure control apparatus 70 and transferred to the filtratestorage vessel 500. One or more filtering units 410, 430 can be providedin parallel in accordance with necessary capacity.

Specifically, when the product is divided into the CNT filtered productand the filtrates via the filtering units 410, 430 connected inparallel, if the filtering unit 410 is subject to pressure to cause itsvalve to be closed, the filtering unit 430 is opened to filter thepurified and cooled product, and at the same time the CNT filteredproduct 413 within the filtering unit 410 are recovered and thefiltrates 411 is transferred to the filtrate storage vessel 500.

Similarly, if the filtering unit 430 is subject to pressure to cause itsvalve to be closed, the filtering unit 410 is opened to filter thepurified and cooled product, and at the same time the CNT filteredproduct 433 within the filtering unit 430 are recovered and thefiltrates 431 is transferred to the filtrate storage vessel 500. Suchoperation of the filtering units is processed repeatedly in analternating manner, which results that the filtering can be accomplishedcontinuously. After the filtering step S911, the product is undergonethrough the product recovery step S913 that recovers the filteredproduct of solid state and then the de-pressurizing step S915 thatde-pressurizes it up to 1 to 10 atm.

More specifically, the product cooled via the cooling step S800 istransferred to the de-pressuring tub 600 and undergone through thede-pressurizing step S931 up to 1 to 10 atm. The de-pressurizing step isprocessed such that the product is firstly de-pressurized up to 10 to100 atm by a capillary de-pressuring apparatus and then finallyde-pressurized up to 1 to 10 atm by a pressure control apparatus 80.

More specifically, the product cooled via the cooling step S800 istransferred to the de-pressuring tub 600 and undergone through thede-pressurizing step S931 up to 1 to 10 atm. The de-pressurizing step isprocessed such that the product is firstly de-pressurized up to 10 to100 atm by a capillary de-pressuring apparatus and then finallyde-pressurized up to 1 to 10 atm by a pressure control apparatus 80.

The continuous method of purifying carbon nanotubes according to thepresent invention is finally performed by recovering the carbon nanotubepurified via the recovering step S933 into the product storage 700.

The continuous apparatus used for purifying the carbon nanotube includesa mixing unit 100 that forms the carbon nanotube solution by allowingthe carbon nanotube to be mixed with the solvent containing water by thecirculation pump; a pre-heater 200 that heats the carbon nanotube liquidmixture formed by allowing the pre-treated carbon nanotube solution tobe mixed with the oxidizer at a temperature of 100 to 370° C. while thecarbon nanotube solution is injected at a pressure of 50 to 400 atm; afirst purifying-reactor 310 in which the liquid mixture is injectedunder the sub-critical water or supercritical water condition processedat a pressure of 50 to 400 atm; a de-pressurizing unit 600 thatde-pressurizes the purified product up to 1 to 10 atm via the coolingapparatus 60 that cools the purified product to a temperature of 0 to100° C.; and a storage vessel 700 that recovers the productde-pressurized by the de-pressurizing unit.

Further, the continuous apparatus of purifying carbon nanotubesaccording to the present invention can further include the secondpurifying-reactor 330 that causes the first purified product purifiedvia the first purifying-reactore to be reacted with the acid solution toremove the inorganic matters.

Further, the continuous apparatus of purifying carbon nanotubesaccording to the present invention is provided with the heat exchanger40 located at a front end of the pre-heater 200, in which the heatexchanger 40 is to exchange heat between the carbon nanotube liquidmixture prior to pre-heating and the purified product.

The continuous apparatus of purifying carbon nanotubes is characterizedin that the de-pressurized apparatus is used with a capillaryde-pressurizing apparatus.

The continuous apparatus of purifying carbon nanotubes further includesthe filtering units 410, 430 which have high-pressure filters having avoid of 0.001 to 10 μm connected in parallel to be operated in aswitching manner. If the void of the high-pressure filter is less than0.00 μm, the purified carbon nanotube blocks the filter so that there isa concern about an energy load, and if it is greater than 10 μm, thefiltering efficiency is eliminated so that there are concerns that thepowder particles of the carbon nanotube are not equal.

The present invention provides the carbon nanotube purified by thecontinuous method.

Hereinafter, embodiments of the present invention will be describedspecifically.

Embodiment 1

After inputting carbon nanotubes of 10 g and distilled water of 990 ginto the mixing unit 100, they are stirred while being circulated by thecirculating pump 10 to produce the CNT solution. The CNT solution isinjected into the pre-heater 200 past the heat exchanger 40 at a flowrate of 13 g/min using the CNT solution high-pressure infusion pump 20and then pre-heated up to a temperature of 220 to 260° C. Then, theoxygen of gas state compressed at a pressure of 245 to 252 atm is mixedwith the CNT solution at a front end of the pre-heater 200 at a flowrate of 0.7 g/min. The CNT liquid mixture is injected into the firstpurifying-reactor 310 under the sub-critical water condition at atemperature of 280 to 310° C. and a pressure of 230 to 250 atm to causethe oxygen to be reacted with the nano carbon, amorphous carbon andalpha carbon present in the CNT liquid mixture, thereby purifying theCNT firstly. Then, the nitric acid of 2.2 M is injected into the secondpurifying-reactor 330 using the acid solution high-pressure infusionpump 50 at a flow rate of 13 g/min and then reacted with the firstpurified CNT liquid mixture purified by the first purifying-reactor 310to remove the inorganic matters such as metal present within the carbonnanotube, thereby purifying the CNT secondly. The purified CNT isconfirmed using a Scanning Electron Microscope SEM and a TransmissionElectron Microscope TEM.

COMPARATIVE EXAMPLE 1

The purified product is obtained by processing the CNT with only a firstpurifying step which is same to the first purifying step of theembodiment 1. The purified CNT is confirmed using a Scanning ElectronMicroscope SEM and a Transmission Electron Microscope TEM.

Embodiment 2

After undergoing a first purifying step that is same to that of theembodiment 1 except that the temperature of the first purifying-reactoris 340 to 360° C., the nitro methane of 2.2 M is injected into thesecond purifying-reactor 330 using the acid solution high-pressureinfusion pump 50 at a flow rate of 13 g/min and oxidized in accordancewith the reaction equation 1 below to cause the CNT solution to bepurified firstly by the nitric acid occurring instantly, and then theCNT solution is purified secondly by removing the inorganic matters suchas metal present carbon nanotube.NO₂CH₃+2O₂---→HNO₃+CO₂+H₂O  [Reaction Equation 1]

Test Method

1. Scanning Electron Microscope SEM

It is a S4800 model from Hitachi. We have dispersed the CNT onto water,dropped them on glass plate, and dried them completely, plated them withplatinum, and then measured it with SEM.

FIGS. 4 a and 4 b are Scanning Electron Microscope pictures of thecarbon nanotube which are not purified according to a comparativeexample 1, in which FIG. 4 a is a hundred thousand-times magnifiedpicture and FIG. 4 b is a fifty thousand-times magnified picture.

FIGS. 5 a and 5 b are Scanning Electron Microscope SEM pictures of thefirst purified carbon nanotube according to the embodiment 2, in whichFIG. 5 a is a hundred thousand-times magnified picture and FIG. 5 b is afifty thousand-times magnified picture.

FIGS. 6 a and 6 b are Scanning Electron Microscope SEM pictures of thecarbon nanotubes which are firstly and secondly purified according tothe embodiment 1, in which FIG. 6 a is a hundred thousand-timesmagnified picture and FIG. 6 b is a fifty thousand-times magnifiedpicture.

As shown from the result of FIGS. 4 a, 4 b to 6 a, 6 b, it will beappreciated that the amorphous carbon component is more purified inFIGS. 5 a and 5 b and the amorphous carbon and the inorganic componentare more purified in FIGS. 6 a and 6 b, as compared with FIGS. 4 a and 4b.

2. Transmission Electron Microscope: TEM

It is a JEM-2100F(HR) model from JEOL. We have dispersed the CNT ontowater, dropped them onto a grid of holic type, dried them completely,and measured them with TEM.

FIGS. 7 a and 7 b are Transmission Electron Microscope pictures of thecarbon nanotube which is not purified according to Comparative example1, in which FIG. 7 a is a hundred thousand times-magnified picture andFIG. 7 b is a fifty thousand times-magnified picture.

FIGS. 8 a and 8 b are Transmission Electron Microscope pictures of thecarbon nanotube which is firstly purified according to Embodiment 2, inwhich FIG. 8 a is a hundred thousand times-magnified picture view andFIG. 8 b is a fifty thousand times-magnified picture view.

FIGS. 9 a and 9 b are Transmission Electron Microscope pictures of thecarbon nanotube which is firstly and secondly purified according toEmbodiment 1, in which FIG. 9 a is a hundred thousand times-magnifiedpicture and FIG. 9 b is a fifty thousand times-magnified picture.

As shown from the result of FIGS. 7 a, 7 b to 9 a, 9 b, it will beappreciated that the amorphous carbon component is more purified inFIGS. 8 a and 8 b and the amorphous carbon and the inorganic componentare more purified in FIGS. 9 a and 9 b, as compared with FIGS. 7 a and 7b.

As described above, the continuous method of purifying carbon nanotubesaccording to the present invention has an advantage in that theproduction process can be shortened since it uses the oxidizer which isnot harmful under the sub-critical water or supercritical watercondition and easy to handle and treat and the carbon nanotube ispurified through the continuous apparatus.

Further, the impurities can be eliminated by injecting the oxidizer viacontinuous sub-critical water or supercritical water process in order toeliminate carbon impurities such as nano carbon, amorphous carbon andalpha carbon contained within the carbon nanotube solution, and the acidsolution is injected into the sub-critical water or supercritical waterto cause the acid to be easily introduced to the inorganic matters,thereby improving the purification rate.

Further, the carbon nanotube purified according to the present inventioncan be obtained in a liquid state or a solid state via the continuousapparatus.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A continuous method of purifying carbon nanotubes, comprising: a)feeding a carbon nanotube solution including carbon nanotube and solventinto a preheater while feeding an oxidizer into the preheater via a heatexchanger to produce a carbon nanotube mixture of the carbon nanotubesolution and the oxidizer and then preheating the resultant carbonnanotube mixture at 100 to 370° C.; b) feeding the carbon nanotubemixture from the step (a) into a purifying reactor under subcriticalwater condition of 50 to 400 atm to obtain a purified product; c)reacting the purified product from step (b) with an acid solution toremove inorganic material; d) cooling down the purified product into 0to 100° C. and depressurizing the purified product into 1 to 10 atm byfeeding the purified product into a cooling down and depressurizing partvia the heat exchanger of the step (a) while carrying out the process ofstep (a); and e) recovering the cooled down and depressurized productfrom the step (d).
 2. The continuous method of purifying carbonnanotubes of claim 1, wherein the carbon nanotube is selected from agroup consisted of a single-walled, a double walled, a multi-walled,roped and a combination of them.
 3. The continuous method of purifyingcarbon nanotubes of claim 1, wherein the solvent used in the step (a) isselected from a group consisting of water, aliphatic alcohol of C₁-C₂₀,carbon dioxide and a combination of them.
 4. The continuous method ofpurifying carbon nanotubes of claim 3, wherein the carbon nanotube iscontained at 0.0001 to 10 parts by weight per 100 parts of the solvent.5. The continuous method of purifying carbon nanotubes of claim 1,wherein the carbon nanotube solution further contains a nitro compoundsof a chemical equation 1:R—(NO_(x))_(y)  [Chemical equation 1] where R is an alkyl group of C₁-C₇or an aryl group of C₆-C₂₀, and x and y are integers of 1 to 3independently.
 6. The continuous method of purifying carbon nanotubes ofclaim 5, wherein the nitro compound is contained at 0.00001 to 3 mol/L.7. The continuous method of purifying carbon nanotubes of claim 1,wherein the oxidizer is selected from a group consisted of oxygen, air,ozone, hydrogen peroxide, nitric acid, nitro compound, nitric acidoxidizer, and a combination of them.
 8. The continuous method ofpurifying carbon nanotubes of claim 1, wherein the oxidizer is containedat 0.001 to 10 equivalent weight in proportion to a carbon equivalentweight of the carbon nanotube within the carbon nanotube liquid mixture.9. The continuous method of purifying carbon nanotubes of claim 1,wherein the acid solution is an aqueous solution containing any one acidselected from a group consisted of nitric acid, phosphoric acid,sulfuric acid, hydrochloric acid, fluoric acid, and a combination ofthem.